The invention concerns the field of semiconductor thin film deposition for photovoltaic applications.
Thin films of copper and indium and/or gallium diselenide and/or disulfide (CIGS) are deposited on a substrate in order to fabricate photovoltaic cells. Such compounds of general formula CuGaxIn1−xSe2−ySy (with x between 0 and 1 and y between 0 and 2) are regarded as highly promising and could constitute the next generation of thin-film solar cells. CIGS semiconductor materials have a direct band gap of between 1.0 and 1.6 eV, which permits strong absorption of solar radiation in the visible range. Record conversion efficiencies of more than 18% have recently been obtained with cells of small surface areas.
CIGS are also referred to as I-III-VI2, referring to the chemical nature of their constituents, where:                the element Cu represents an element from column I (column 1B of the periodic table),        the element In and/or the element Ga represent elements from column III (column 3B of the periodic table), and        the element Se and/or the element S represent an element from column VI (column 6B of the periodic table).        
There are therefore approximately two column VI atoms per column I atom and per column III atom in the monophase domain around the I-III-VI2 composition of the CIGS.
The CIGS layers used for photovoltaic conversion need to have a p-type semiconductor character and good charge transport properties. These charge transport properties are favored by good crystallinity. The CIGS thus need to be at least partially crystallized in order to have sufficient photovoltaic properties for their use in the fabrication of solar cells. Crystallized CIGS compounds have a crystallographic structure corresponding to the chalcopyrite or sphalerite systems, depending on the deposition temperature.
When they are deposited at a low temperature (precursor deposition), CIGS thin films are weakly crystallized or amorphous, and annealing of the layers has to be carried out by supplying heat in order to obtain an improvement of the crystallization of the CIGS and satisfactory charge transport properties.
At the temperatures which allow at least partial crystallization of the CIGS, however, one of the constituent elements of the CIGS (principally selenium Se) is more volatile than the other elements. It is therefore difficult to obtain crystallized CIGS with the intended composition (close to the I-III-VI2 stoichiometry) without adding selenium for the annealing of the precursor layer.
Furthermore, in order to obtain a p-type semiconductor character (conduction by holes) the composition of the layers should have a slight deviation from the I-III-VI2 stoichiometry in favor of the VI element.
In the fabrication of CIGS thin films for photovoltaic applications, therefore, the prior art embodiments involve annealing the precursor deposits in the presence of a selenium excess in the vapor phase.
The best photovoltaic conversion efficiencies (more than 17%) have been obtained from CIGS when preparing thin films by evaporation. Evaporation is an expensive technique which is difficult to use on the industrial scale, however, particularly because of nonuniformity problems with the thin-film deposits over large surface areas and a low efficiency of using the primary materials.
Cathodic sputtering is more suitable for large surface areas, but it requires very expensive vacuum equipment and precursor targets. The term “precursors” means intermediate compounds whose physicochemical properties are distinct from those of CIS (or CIGS) and make them incapable of photovoltaic conversion. They are initially deposited in a thin-film form, and this thin film is subsequently processed in order to form the intended CIGS deposit.
Electrochemical deposition offers an advantageous alternative. The difficulties which are encountered, however, relate to controlling the quality of the electrodeposited precursors (composition, morphology) and processing them with a view to providing adequate electronic properties for the photovoltaic conversion. Several approaches have been proposed in order to overcome these difficulties, including the following:                separate or sequential electrodeposition of the Cu then In precursors, followed by addition of Se (a step referred to as “selenization”), as described in U.S. Pat. No. 4,581,108;        electrodeposition of (Cu, In) binaries in the presence of an Se suspension, as described in U.S. Pat. No. 5,275,714.        
This is because it is easier to apply a single precursor at the time.
More recent developments (U.S. Pat. No. 5,730,852, U.S. Pat. No. 5,804,054 ) propose electrodeposition which is equivalent to depositing a layer of precursors with the composition CuxInyGazSen (with x, y and z between 0 and 2, and n between 0 and 3) by using a pulsed current method. The electrodeposition is followed by a step of evaporating the elements In, Ga and Se in order to increase their levels compared with the electrolyzed layer.
As regards “pure” electrodeposition, that is to say electrodeposition without an evaporation step and with the I-III-VI2 stoichiometry, the best efficiencies are about 6 to 7% as indicated in the following publications:                GUILLEMOLES et al., Advanced Materials, 6 (1994) 379;        GUILLEMOLES et al., J. Appl. Phys., 79 (1996) 7293.        
These publications furthermore indicate that better results are obtained when the annealing is carried out under selenium vapor pressure, at temperatures in excess of 450° C., in a vacuum. Conventional annealing is then performed in a diffusion furnace, under elementary Se pressure. Such annealing, however, is relatively time-consuming (of the order of one to a few hours).
Document U.S. Pat. No. 5,578,503 describes a two-step method which firstly involves deposition by cathodic sputtering then rapid annealing (with lamps) of the precursors deposited in this way. In particular Cu, In and Se precursor elements are deposited separately in an elementary form (Cu(0), In(0) and Se(0)) or in the form of binaries (such as In2Se3). The structure initially deposited before annealing is thus essentially heterogeneous and is in the form of a multiplicity of successive sheets (stacks of Cu0/In0/Se0 or Cu0/In2Se3, or a combination of the two) in the thickness direction of the layer. This mixture of precursors subsequently undergoes rapid annealing, comprising a temperature rise followed by a holding time needed to homogenize the CIS layer. When it is deposited by sputtering and has a heterogeneous structure in sheets, however, the thin film is not good at withstanding abrupt rises in temperature, especially in mechanical terms. Since the thermal expansion coefficient of the layer is spatially inhomogeneous, delamination problems occur with this layer during the annealing step. Although advantageous, such a procedure is therefore not yet completely satisfactory.
More generally, the methods of deposition by evaporation or sputtering use sources which commonly consist of pure elements, or sometimes binaries but rarely ternaries. A difficulty arises when carrying out such methods. It involves transfer of the elements from the source to the substrate. This transfer is not the same for all the elements, and the evaporation speed or sputtering rate may differ from one element to another. At a high temperature, in particular, the vapor pressures of the elements (their volatility) may differ significantly. This effect is commensurately more problematic when there are a large number of elements in the alloy to be obtained (ternary CIS, quaternary CIGS, etc.).
It is an object of the present invention to improve the situation.